How do you figure? I always had in my mind that FU money was the amount you needed to quit immediately and be fine forever, which also sounds pretty much like FIRE. Granted, my mental model of FU money is not cash in the bank but passive cashflow which is probably coming from investments FIRE-style, which is maybe where the difference is.
If you have FU money you don't need to run the numbers.
That's how I look at it.
1. FIRE tend to mean sufficient money in the bank/money from passive incomes that let you live a fairly frugal lifestyle. See the Mr. Money Mustache stuff that various people are especially taken with. It doesn't tend to mean Retire Early and take expensive trips, buy nice cars, pursue expensive hobbies, buy second homes
2. FU money, on the other hand, tends to mean you don't need to work and can have enough money to do virtually anything you want to. There are degrees of course once you're talking things like private jet ownership.